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Microwave Communication Links
Microwave Communication Links
http://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/wireless.htm#wp1020742
Since the beginning of development of microwave wireless transmission equipment, manufacturers and operators have tried to mitigate the effects of reflected signals associated with signal propagation. These reflections are called multipath. In real-world situations, microwave systems involve careful design to overcome the effects of multipath. Most existing multipath mitigation approaches fall well short of the full reliable information rate potential of many wireless communications systems. This section discusses how to create a digital microwave transmission system that not only can tolerate multipath signals, but that also can actually take advantage of them. Digital microwave systems fall into two categories: wavelengths less than 10 GHz and wavelengths greater than 10 GHz (referred to as millimeterwave). Several bands exist below 10 GHz for high-speed transmissions. These may be licensed bands, such as MMDS (2.5 GHz), or unlicensed bands, such as U-NII (5.7 GHz). Bands that are below 10 GHz have long propagation distances (up to 30 miles). They are only mildly affected by climatic changes such as rain. These frequencies are generally not absorbed by objects in the environment. They tend to bound and thus result in a high amount of multipath. Bands over 10 GHz, such as 24 GHz, LMDS (28 GHz), and 38 GHz, are very limited to distance (less than 5 miles). They are also quite susceptible to signal fades attributed to rain. Multipath tends not to be an issue because the transmission distances are less and because most of the multipath energy is absorbed by the physical environment. However, when these frequencies are used in highly dense urban areas, the signals tend to bounce off objects such as metal buildings or metalized windows. The use of repeaters can add to the multipath propagation by delaying the received signal. |
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This intel was contributed by Infinite
Infinite
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May, 2012
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